PTL 1 describes that in a solid-oxide fuel cell system, when a gas in a reformer contracts due to a temperature decrease of the reformer, a raw material is supplied to the reformer.
PTL 2 describes that in a solid-oxide fuel cell system, an off gas that is a mixture of a fuel gas and an oxidizing gas which have not been used in a solid-oxide fuel cell is subjected to a combustion treatment.
PTL 3 describes that after electric power generation of a solid-oxide fuel cell system is stopped, a fuel gas and air are continuously supplied to a fuel cell stack.